(1) Field of the Invention
The present invention relates to a tidal current meter that measures the velocity of a tidal current at a predetermined depth of water such as the ocean or a lake.
(2) Description of the Related Art
A well-known tidal current meter described, for instance, in Japanese Unexamined Patent Application Publication No. 2011-203276 transmits an ultrasonic wave into water, receives a wave reflected, for example, from clutter, and measures the velocity of a tidal current at a predetermined depth in the water in accordance with the Doppler shift frequency of the reflected wave.
A conventional tidal current meter is configured so that three oscillators are mounted, for example, on the bottom of a ship and positioned 120 degrees apart from each other when the surface of water is viewed from above. The tidal current meter is configured so that the three oscillators transmit an ultrasonic wave in directions 120 degrees apart from each other at a fixed depression angle θ (a tilt angle or other angle formed by the direction of ultrasonic wave transmission and the surface of water (horizontal plane) on which the ship is floating) in order to let each of the oscillators receive a wave reflected from clutter (for example, plankton) existing at a target depth for measurement.
Then, in accordance with the Doppler shift frequency of the reflected wave received by each of the oscillators, the tidal current meter calculates the velocity of a tidal current at the target depth for measurement.
In order to be able to measure the velocity of a tidal current at a deep place, the conventional tidal current meter causes the ultrasonic wave to reach the deep place by using a fixed depression angle θ of approximately 60 degrees. In this instance, however, it is difficult to measure the velocity of a tidal current at a small depth (hereinafter may be referred to as a shallow place) for the following reason.
When the conventional tidal current meter transmits the ultrasonic wave from the oscillators, a wave reflected from clutter at a shallow place reaches the oscillators immediately. In order to enable the oscillators to receive the reflected wave, therefore, it is necessary to reduce the pulse width of the ultrasonic wave transmitted by the oscillators (shorten the time of ultrasonic wave transmission) or reduce the width of frequency analysis required for calculating the Doppler shift frequency (decrease the number of data used for frequency analysis). However, such pulse width reduction and frequency analysis width reduction will decrease the frequency resolution, resulting in the failure to properly measure the Doppler shift frequency. This makes it difficult to measure the tidal current at a shallow place.